Field of the Invention--The present invention relates to the production of hydrogen, and more particularly to reducing the carbon monoxide level in a hydrogen stream using a shift converter.
Description of the Prior Art--In one method for producing hydrogen a hydrocarbon feedstock and steam are fed into a steam reforming reactor containing a nickel catalyst. In the reactor, the hydrocarbons are converted to hydrogen, carbon dioxide and carbon monoxide.
If the hydrogen produced is to be used in a fuel cell wherein the electrode catalyst is platinum, it may be desirable to reduce the carbon monoxide level in the reactor effluent to very low levels to minimize poisoning of the platinum. A shift converter may be used for this purpose. In the shift converter water in the reactor effluent combines with the carbon monoxide to produce hydrogen and carbon dioxide as represented by the following equation: EQU H.sub.2 O+CO.fwdarw.H.sub.2 +CO.sub.2
A typical shift converter catalyst is iron oxide stabilized with chromia.
Most raw hydrocarbon feedstocks contain some sulfur which, in high enough concentrations, poisons the nickel steam reforming catalyst if reforming is conducted at temperatures below about 1500.degree. F. When light hydrocarbon feedstocks are used, such as natural gas, naphtha or L. P. gas (propane), the sulfur contained therein is reduced to acceptable levels upstream of the reactor by converting the organic sulfur in the feedstock to hydrogen sulfide using a hydrodesulfurizer, and subsequently removing the H.sub.2 S in an adsorbent bed of zinc oxide and/or charcoal. Commonly owned U.S. Pat. Nos. 3,476,535 and 3,480,417 show fuel cell systems with desulfurizing means upstream of a steam reforming reactor. With heavier feedstock, such as No. 2 fuel oil, the sulfur content of the fuel may be so high and the sulfur compounds so unreactive that it is not practical or perhaps not desirable to remove the sulfur upstream of the reactor. Instead the reactor may be run at a much higher temperature, such as greater than 1500.degree. F. whereby the sulfur does not completely poison the catalyst and is converted to hydrogen sulfide within the reactor. It is known, however, that high levels of sulfur (in the form of hydrogen sulfide) in the shift converter feed may react with the iron oxide-chromia shift conversion catalyst and convert the iron oxide to iron sulfide which is only 50% as active, in terms of shift converting, as iron oxide. If this is the case, the practice has been to either remove this hydrogen sulfide upstream of the shift converter or to use, for example, twice the amount of shift catalyst to compensate for the 50% reduction in activity.
Assuming the H.sub.2 S concentration is not so high as to result in the conversion of iron oxide to iron sulfide, it is taught that H.sub.2 S is, at best, a mild poison to the iron oxide catalyst. For example, the Catalyst Handbook (1970) distributed by Springer-Verlay N.Y. Inc., N.Y. (Library of Congress Catalogue #70-121128) states on page 103 that less than 1 ppm H.sub.2 S, by volume, in the inlet gas results in increased catalytic activity, but 50 and 100 ppm H.sub.2 S results in normal and decreased activity, respectively. Thus, to reduce the amount of carbon monoxide in a hydrogen sulfide containing gas stream to a preselected level, it has been the practice to use an amount of catalyst in excess of the amount which would normally be required if there were no hydrogen sulfide in the gas stream, the additional amount of catalyst needed being determined by the expected reduction in catalyst activity.